In order to bore relatively small hole in a workpiece at high speed by means of electroerosion using electrical discharges, a relatively long tubular electrode of a few millimeters or less in diameter is widely used. A machining current having a high current density (e.g., 1000 A/cm.sup.2 or more) is applied to the machining gap while the gap is flushed with a working fluid from the tubular electrode at a relatively high pressure (e.g., 20 kg/cm.sup.2 or more). In this type of electroerosion boring apparatus, the workpiece can be machined at a relatively high feed rate (e.g., 20 mm/min or more) which is several to more than ten times as large as that of the usual die-sinking electric discharge machine when machining at a small wear rate of electrode. Due to such high speed machining performance, high speed electrical discharge machining is now widely applied for purposes other than small hole drilling, for example, for machining small-sized parts.
Japanese Patent Application Laid-Open No. 172423/1985 discloses an electrical discharge boring apparatus for attachment to a wire-cut electric discharge machine (EDM) using a tubular electrode through which a working fluid flows into a machining gap to produce a so-called initial hole for threading a wire electrode in a workpiece.
In electrical discharge hole boring operations, the workpiece material is removed by electrical discharges which repeatedly melt minute particles of metal from the workpiece. This removed metal forms chips when cooled by the machining fluid. These chips may adhere to the peripheral edge of the opening of a machined hole or depression and form a fused adhesive deposit, resulting in degradation of the finish quality of the workpiece. Therefore, the fused or sputtered chips have to be removed by additional operations, which are troublesome, particularly when machining small-sized parts.
It is well known that the build-up of the fused adhesive deposit is dependent on the discharge machining parameters. When boring a small hole or depression using a high feed rate and large machining energy, the build-up of the fused adhesive deposit is large in comparison to slower, lower-energy machining. If the discharge machining energy is lowered to reduce the build-up of the adhesive deposit, the high-speed performance, which is the greatest advantage of this type of electrical discharge boring, is lost.
Conventionally, as alluded to above, it was necessary to remove the fused adhesive deposit after the completion of the high-speed electrical discharge boring using other processes, thereby resulting in reduced machining efficiency and increased cost.